(1) Field of the Invention
The present invention relates to gas turbine engines, and more specifically to systems and components for injecting a secondary fluid stream into a primary fluid stream with improved aerodynamic efficiency.
(2) Description of the Related Art
Gas turbine engines operate according to a continuous-flow Brayton cycle where ambient air is compressed, fuel is added, the fuel-air mixture is combusted, and the products of combustion are expanded through an annular duct in a turbine. Alternating stages of stationary turbine vanes and rotating turbine blades transfer the energy of the expanding gas in the duct to a shaft, which in turn powers a large diameter fan, a gearbox or a generator. The residual energy of combustion may be discharged from the engine as thrust. Gas turbine engines are frequently used to power aircraft, heavy-duty vehicles, ships and electrical generators.
Within a typical gas turbine engine, compressed ambient air is directed into two fluid streams: a primary and a secondary. The primary fluid stream is mixed with fuel and combusted, and the secondary fluid stream is used for cooling the vanes, blades and other critical components in the turbine. The secondary fluid stream is directed radially about the combustor and annular duct to cool the critical turbine components before being introduced into the primary fluid stream. The secondary fluid stream is radially introduced into the axially and tangentially directed primary fluid stream, which leads to aerodynamic mixing losses.
Aerodynamic losses in the primary fluid stream also occur at the juncture of vane and blade airfoils and the duct's peripheral endwalls. At the endwalls, the primary fluid stream is dominated by a vortical flow structure known as a horseshoe vortex. The vortex forms as a result of an endwall boundary layer, which separates from the endwall as the primary fluid stream approaches the leading edges of the airfoils. The separated stream reorganizes into a horseshoe vortex, which leads to an aerodynamic loss oftentimes referred to as “secondary” or “endwall” loss. Endwall loss attributes to as much as 30% of the loss in a row of airfoils. A further description of the horseshoe vortex, the associated fluid dynamic phenomena and geometries for reducing endwall losses can be found in U.S. Pat. No. 6,283,713 to Harvey, et al. and in Sauer et al., “Reduction of Secondary Flow Losses in Turbine Cascades by Leading Edge Modifications at the Endwall”, ASME 2000-GT-0473.
In prior art reference U.S. Pat. No. 4,311,431 to Barbeau, tangentially angled holes in a shroud minimize undesirable air turbulence at a blade tip. In another reference, U.S. Pat. No. 6,481,959 to Morris, et al., ingestion-inhibiting jets below the platform prevent ingestion of the primary fluid stream into a radial gap. In additional references, U.S. Pat. No. 6,089,822 to Fukuno and U.S. Pat. No. 6,761,529 to Soechting, et al., plural trailing edge flow paths cool the trailing edge of the platform before being discharged at the trailing edge.
It is widely understood that minimization or elimination of aerodynamic losses in a turbine improves efficiency and greatly reduces fuel consumption. A commercial airline may spend up to 30% of its operating expenses on fuel, so any reduction in fuel consumption directly benefits the commercial airline industry as a whole. Although the benefits of the prior art systems are acknowledged, it is desirable to reduce both the mixing and endwall losses with a single injection system.